In recent years, a Giant Magnetoresistance (GMR) effect element comprising a multilayer film of a ferromagnetic layer/non-magnetic metal layer and a ferromagnetic tunnel junction (MTJ) comprising a ferromagnetic layer/insulator layer/ferromagnetic layer have attracted attention for new magnetic field sensors and non-volatile magnetic random access memory (MRAM) cells. With respect to the GMR, there have been known a CIP-GMR of a type such that an electric current flows within the plane of the film and a CPP-GMR of a type such that an electric current flows in the direction perpendicular to the plane of the film. The principle of the GMR resides mainly in spin-dependent scattering at the interface between the ferromagnetic layer and the non-magnetic layer, but spin-dependent scattering in the ferromagnetic material (bulk scattering) also contributes to the principle. For this reason, generally, in the multilayer film, the CPP-GMR to which the bulk scattering is expected to contribute is larger than the CIP-GMR. For the practical use a spin valve type device has been used, in which an antiferromagnetic layer is disposed in the vicinity of one of the ferromagnetic layers to pin the spin of the ferromagnetic layer.
On the other hand, in the MTJ, by controlling the magnetization configurations of two ferromagnetic layers to be parallel or antiparallel to each other using an external magnetic field, a so-called tunnel magnetoresistance (TMR) effect in which the tunnel currents in the direction perpendicular to the planes of the films are different from each other can be obtained at room temperature. The TMR ratio in the tunnel junction depends on a spin polarization P at the interface between the ferromagnetic material and insulator used, and it is known that when spin polarizations of two ferromagnetic materials are respectively taken as P1 and P2, TMR ratio is generally given by the following Julliere's formula (1):TMR=2P1P2/(1−P1P2)  (1)
A ferromagnetic material has a spin polarization P which satisfies the relationship: 0<P≦1. Also for the MTJ, a spin-valve structure in which an antiferromagnetic layer is kept adjacent to one ferromagnetic layer thereby pinning the spin of the ferromagnetic layer is in practical use.
As can be seen from the formula (1), when using a ferromagnetic material having a spin polarization P=1, an infinitely large TMR is expected. A magnetic material having P=1 is called a half-metal, and from the band calculations already made, oxides, such as Fe3O4, CrO2, (La—Sr)MnO3, Th2MnO7, and Sr2FeMoO6, half Heusler alloys, such as NiMnSb, full-Heusler alloys having an X2YZ-type composition and having an L21 structure, such as Co2MnGe, Co2MnSi, and Co2GrAl, and the like are known as half-metals. Above all, Co-based full-Heusler alloys with X=Co have a high Curie point and are therefore most expected for applications. Full-Heusler alloys have an disordered structure of a B2 or A2 (body-centered cubic) structure, in addition to the L21 ordered structure. In general, it is known that B2 structure also becomes a half-metal like L21 structure, however, A2 structure destroys half-metallicity. Heat treatment is necessary for obtaining the L21 and B2 structures; and for obtaining the B2 structure, in general, the substrate must be heated up to 200° C. or higher, or must be post-annealed at a temperature above 300° C. after film deposition at room temperature, though depending on the composition thereof. For obtaining the L21 structure, a higher temperature than that is needed.
Amorphous Al oxide (AlOx) films and (001)-oriented MgO films have been used for barrier materials in MTJ. The former are formed through film formation of Al metal according to a sputtering method followed by oxidation according to a method of plasma oxidation or the like, and it is well known that their structure is amorphous (Non-Patent Document 1). In the AlOx barrier, the interface roughness between the ferromagnetic layer and the barrier layer is generally large and the TMR variation is large, and in addition, a large TMR is difficult to be obtained; and therefore, recently, a crystalline MgO barrier has become much used. MgO barrier can be made by a method of direct sputtering from an MgO target, or a method of evaporation with an MgO shot using electron beams. However, in MTJ with a full-Heusler alloy, in general, a high-quality MgO barrier is difficult to be obtained; and therefore, an electron beam evaporation method is applied.
Up to date, as the Co-based half-metal full-Heusler alloys used for the magnetic layer in MTJ, there are known Co2MnSi, Co2MnGe, Co2Cr0.6Fe0.4Al, Co2FeAl0.5Si0.5, etc. Of those, the MTJ including Co2FeAl0.5Si0.5 has the largest TMR at room temperature, in which the MgO barrier is formed using an electron beam evaporation method; and the room-temperature TMR of the MTJ having a spin-valve structure is 220% (Non-Patent Document 2).
As another method of obtaining large TMR not using a half-metal, there is known a method of utilizing a coherent tunnelling effect. An MgO barrier is used and its crystal is (001)-oriented, and the upper and lower ferromagnetic layers arranged to dispose the barrier therebetween are also (001)-oriented epitaxial tunnel junctions. In such a case, there occurs a coherent tunnelling effect of such that a Δ1 band electron having a large tunnelling transmission essentially contributes toward tunnel. It is known that, in this case, on the Fermi level of the ferromagnetic layer, when the Δ1 band exists in one spin band (for example, majority spin band) but does not exist in the other spin band (for example, minority spin band), TMR is greatly enhanced owing to the coherent tunnelling effect (Non-Patent Document 3). Up to now, as ferromagnetic materials with which great enhancement of TMR owing to the coherent tunnelling effect thereof has been reported, there are known bcc-crystalline structured Fe, Co, Fe—Co alloys and CoFeB alloys.
It has been theoretically pointed out that the coherent tunnelling effect is also effective for Co-based full-Heusler alloys, such as Co2MnSi (Non-Patent Document 4); and in fact, in MTJ including Co2MnSi, the matter has been observed (Non-Patent Document 5) when MgO barrier was fabricated using an electron beam deposition. However, in case where a Co-based full-Heusler alloy is used as a ferromagnetic layer material, in general, the lattice misfit thereof with MgO is large, and therefore, especially when the MgO barrier is formed using a sputtering method, there occur many dislocations inside the MgO barrier and in the interface between the ferromagnetic layer and the MgO layer, and therefore a high-quality tunnel barrier could not be obtained; and in addition, the Co-based full-Heusler alloy structure on the MgO barrier is often a disordered structure. In such a case, owing to the formation of the disordered structure at the interface, the lattice periodicity is broken and the tunnelling electron momentum in the direction perpendicular to the film surface could not be conserved, and therefore, the TMR enhancement owing to the coherent tunnelling effect as theoretically pointed out could not be observed. In other words, the TMR enhancement owing to the coherent tunnelling effect could not always be observed for all Co-based full-Heusler alloys.
The MTJ has currently been practically used in a magnetic read head for hard disk and a non-volatile magnetic random access memory MRAM. In the MRAM, MTJs are arranged in a matrix form and an electric current is allowed to flow a separately provided wiring to apply a magnetic field to the MTJ, so that two magnetic layers constituting each MTJ are controlled to be parallel or antiparallel to one another, thus recording data of 1 or 0. The recorded data is read utilizing a TMR effect. Recently, so-called spin-transfer magnetization switching has become important, which is for switching the MTJ magnetization by injection of spin polarization current; and one problem is how to reduce the critical current density necessary for the magnetization switching. The critical current density is smaller when the tunnel spin polarization of MTJ is larger; and use of MTJ with a larger TMR is desirable. In addition, the critical current density is proportional to the damping constant α of a ferromagnetic material, and therefore, a ferromagnetic material having a smaller α is desirable. Specifically, an MTJ including a ferromagnetic material having a small α as the electrode therein and capable of providing a large TMR is suitable for MRAM. On the other hand, as spintronics devices in future, a technique of spin injection into semiconductor via barrier has become much important in the field of spin MOSFET and spin transistors. In these fields, high-efficiency spin injection into semiconductor is needed, for which earnestly desired is a ferromagnetic material capable of gaining a large spin polarization current and a small critical current density for spin-transfer magnetization switching.
It is known that a Co-based full-Heusler alloy has a smaller a as compared with ordinary Fe-Co alloys (Non-Patent Document 6); and accordingly, desired is development of a Co-based full-Heusler alloy capable of providing a large TMR at room temperature for spintronics devices. Above all, Co2FeAl has a smallest α (Non-Patent Document 6), and therefore, its use is desired; however, since the alloy is not a half-metal (Non-Patent Document 7), a large TMR is not expected for it.